Developmental changes induced by the administration of N-ethyl-N-nitrosourea leading to final emergence of brain tumors will be studied in the brains of the rats. In one series of studies, brains obtained from the animals receiving the carcinogen on days 19, 20 or 21 of gestation will be studied in a developmental sequence to analyze the progressive cellular changes underlying the growth of the experimental brain tumors. In another series of experiments the problem of differential susceptibility of embryos of different gestational stages to the carcinogen will be studied. These also will be conducted in a developmental manner. Analysis of the material will be performed at light and electron microscopic levels.